decrypt.py

key = "1sW28zp7CWv5TtGjlQpDHHG4Cbr9v36fG5o4f74LsKg=".decode("base64")
tag = "OBA7UU/Rd9j0Zn+9korAyQ==".decode("base64")
initialization_vector = "O2QAL8AYQB+qbre8".decode("base64")
encrypted_message = "7YS1aTE=".decode("base64")

from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.backends import default_backend

decryptor = Cipher(
    algorithms.AES(key),
    modes.GCM(initialization_vector, tag),
    backend=default_backend(),
).decryptor()

message = decryptor.update(encrypted_message) + decryptor.finalize()
print "message:", message